Hi krass, and yes like OSTexo mentions above, the problem is that there are some teeny tiny little slivery surfaces in that area, if you zoom in very closely you can see that there's a kind of little tiny slot or shelf in the side of your shape. It probably came from doing booleans with some pieces which were not quite aligned with one another:






There's a little tiny "roof" like piece at the top of that slot, and it does not run the whole length so it may have been 2 pieces that were at some slight angle to one another when they were booleaned together or something like that.

It should probably be possible to repair that area of the model by doing some "utrim" and retrim type techniques, I will try to help do that for you a bit later today. If you want to try it yourself there is a tutorial here that describes the general techniques to use for object repair:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

- Michael